Luminaries having arrays of coloured LEDs, also known as RGB LED luminaries, such as white light emitting luminaries or colour tunable luminaries, are of interest for several reasons. For example, they are low cost and efficient, and the chromaticity of their light output is adjustable.
However, the light output of the LEDs varies from LED to LED, as well as over the lifetime of each LED. Additionally, the light output of the LEDs varies inversely with temperature, and the variation is different for different colours. Many solutions for achieving a stable light output from such luminaries have been tried. In those solutions different kinds of feedback control have been introduced. Thus, the light output is detected and the detections are used for controlling the energising of the LEDs. One prior art solution is disclosed in U.S. Pat. No. 6,127,783, where a white light emitting luminaire has an electronically adjusted colour balance. The luminaire includes a plurality of LEDs in each of the colours red, green and blue, with a separate power supply for each colour and a photosensor array in the form of a photodiode that is arranged to detect the light output of all the LEDs. The light output of each colour is measured by an electronic control circuit, which turns off the LEDs for the colours not being measured in a sequence of time pulses. The measured light output for each colour is compared to a desired output, which may be determined by user inputs, and corrections to the current for each colour are made accordingly.
The thus provided optical feedback control of the LED light require time pulsed measurements to measure the light output for each individual LED light colour, or even for each LED. This is due to the fact that if all LEDs are on the mixed light output of the luminaire is reflected to the photosensor array. The time pulsing is time consuming and significantly limits the maximum output of the luminaire, in particular for luminaries containing a large number of LEDs.